This proposal seeks to procure a mechanically sound and technically refined transmission electron microscope. The major users of the proposed instrument conduct various innovative NIH supported research programs which employ transmission electron microscopy either as a primary research tool or as a vital component of a multifaceted scientific approach. The continued development and expansion of the major users' research efforts coupled with the need for more contemporary ultrastructural capabilities clearly support the need for a new and refined research instrument. Additionally this need is further attested to by the deplorable state of the currently available transmission electron microscopes. In that the major users identified in this proposal have all made significant contributions to their respective fields of endeavor, it is apparent that the procument of a new electron microscope should greatly facilitate their efforts and lead to both an increased degree of productivity and an enhanced level of scientific and intellectual achievement. We believe that at present we are entering into a crucial period in regard to the functional status of the current electron microscopic facility and in our estimation our advancement or decline directly hinges on our ability to obtain the requested instrument. In relation to multiple criteria the proposed Philips EM 410 transmission electron microscope appears as a most suitable and costeffective instrument.